State of the art pneumatic rubber tires are typically prepared with a rubber tread on a toroidal carcass having a sidewall which extends between and connects opposite sides of the tread to opposed spaced apart wire beads. As a tire is driven over pavement it is flexed continuously, the higher the load carried by the tire and the higher its speed, the greater the strain imposed which causes higher stress during dynamic flexing. A cut in the sidewall tends to grow faster as the strain and resultant stress levels increase. In addition, a sidewall of a tire used on a typical automobile is subjected to weathering, and scuffing against curbs when the car is being parked. The combination of high strain and resultant high stress accelerates damage caused by heat buildup and ozone degradation.
Though such degradation also takes its toll on the tread, tear resistance of a tire's tread is of greater importance and even a small improvement thereof is deemed a substantial contribution to the tread's performance. In U.S. Pat. No. 6,046,266 Sandstrom et al teach an improved tread composition containing precipitated silica and a combination of trans-1,4-and cis-1,4-PBD but did not suggest that the substitution of any particular cis-1,4-PBD might contribute any particularly desirable property.
To obtain a desirable balance of properties the art has modified both the NR and SR components in a blend, as well as the relative amounts used of each, and the blend contains various antiozonants/antioxidants (hereinafter, together, “antidegradants”), fillers and curing agents. Choosing the best components for a sidewall recipe is complicated by the requirement that not only they be co-curable in a particular range of temperature, but also that they be compatible when cured. Over the years, improvements have made the modern high speed passenger tire reliable, rugged and affordable. Because fatigue performance of current sidewall compounds is already highly satisfactory, the ability to improve sidewall performance further is not easily achieved.
U.S. Pat. No. 5,451,646 to Castner disclosed that the use of p-styrenated diphenylamine or hexadiene effectively reduced the molecular weight of the cis-1,4-polybutadiene (“cis-1,4-PBD”) produced with a conventional organonickel-based “catalyst package”. Unlike with cis-1,4-PBD produced by conventional organometal catalysts, a cis-1,4-PBD produced with p-styrenated diphenylamine as the molecular weight reducer in the catalyst package has produced unexpectedly good performance characteristics in a sidewall. It is believed that an arylamine affects the structural activity of organonickel-based catalyst packages which are a combination of an organonickel compound, an organoaluminum compound and a fluorine-containing compound, as disclosed in U.S. Pat. No. 4,983,695 to Kuzma et al. Other organometal-based catalyst packages may be modified to produce polymer chains of cis-1,4-PBD having a more dendritic (branched) structure than those produced with hexadiene.
Though the high cis-1,4-PBD produced with the amine or hexadiene modifiers had a cis-content in excess of 95 percent, there is no suggestion that the molecular architecture of the PBDs was either similar or different. Castner disclosed he found it unnecessary to oil-extend the '646 cis-1,4-PBD to improve its processability, indicating it processed as if it had a lower molecular weight than other high cis-1,4-PBDs. One skilled in the art would know that a lower molecular weight polymer will process more easily than the same polymer having a higher molecular weight. Castner did not suggest that either the polydispersity or the branched chain structure of the mass of one high cis-1,4-PBD was different from that of another; or that the properties of a compounded rubber would depend upon the molecular architecture of the cis-PBD produced; or that the molecular architecture had a critical influence in the compound; or that the molecular architecture was controlled by the organometal-based catalyst package with which the cis-PBD was produced.
Therefore one would not expect that a particular high cis-1,4-PBD would have a substantially different compounding effect compared with that of any other high cis-1,4-PBD of essentially the same molecular weight but different molecular architecture, in particular, its dendritic structure, its hydrodynamic volume and its polydispersity. In particular, it was unexpected that a first high cis-1,4-PBD having essentially the same Mooney viscosity as a second high cis-1,4-PBD would provide worse performance characteristics than the second, mainly because the second had a lower Mn, a higher polydispersity and a greater degree of branching.
U.S. Pat. No. 5,244,028 to Segatta et al. teaches that a precipitated silica filler having a BET surface area of between 100 and 250 square meters per gram and a pH in the range 4.0 to 6.5 improves the properties of conventional antidegradants in a sidewall composition, regardless of which synthetic rubbers are used in the composition. Segatta et al. disclose that mixtures of NR and PBD may be used, as well as mixtures of the various types of PBD including a high cis-1,4-PBD wherein at least 90 percent of its butadiene repeat units are a cis-1,4-isomeric structure, and other synthetic rubbers such as medium vinyl PBD having from 40 to about 60 percent 1,2-vinyl repeat units, and trans-1,4-PBD having at least 65 percent trans-1,4 repeat units. It focussed the effect of a particular filler but failed to suggest that any particular high cis-1,4-PBD might inculcate better performance characteristics than another. They could not have known that a nickel-based catalyst package in which an arylamine was used as a molecular weight reducer, produced a relatively low molecular weight high cis-1,4-PBD which had a higher polydispersity and higher degree of branching than others of comparable viscosity. The degree of branching is measured in a 0.1 percent concentration of polymer in a solution of tetrahydrofuran (THF), as a ratio of light scattering to refractive index. The light scattering and refractive index measurements are made at the outlet of a GPC column. The results are corroborated by measurements of solution viscosity in a solvent such as THF or toluene, such solvent molecules having a higher affinity for high cis-1,4-PBD molecules than a θ (theta) solvent.
Since the branched chain structure of high cis-1,4-PBD molecules of approximately the same Mooney viscosity would be expected to have substantially the same molecular weight, irrespective of the process by which each was prepared, there was no reason in the prior art to expect that a particular high cis-1,4-PBD might have a structure which was so different as to change both, the properties and the processing characteristics of the sidewall compound, both substantially; the art failed to recognize that the fatigue performance characteristics of a sidewall compound could be affected by tailoring the molecular architecture of the synthetic rubber component.